Blue light emitting devices such as a light emitting diode (LED) and laser emitting blue light were developed and white light emitting devices using the blue light emitting devices have been put into practical use. The white light emitting devices have excellent light emission efficiency and long product's life. Because of these advantages, the white light emitting devices are expected to be used in a wide variety of fields including backlight of liquid crystal displays and lighting such as indoor lighting and automobile headlight. On the other hand, the brightness and color reproducing and rendering properties of the white light emitting device have been desired to be further improved.
As white light emitting devices, the following three types are known.    1. A first type of white light emitting device has a blue light emitting device in combination with a fluorescent material emitting a complementary blue upon receipt of light emitted from the blue light emitting device.    2. A second type of white light emitting device has a blue light emitting device in combination with a red emitting fluorescent material and a green emitting fluorescent material, which emit fluorescence upon receipt of light emitted from the blue light emitting device.    3. A third type of white light emitting device has an ultraviolet light emitting device, a blue emitting fluorescent material, a green emitting fluorescent material and a red emitting fluorescent material, which emit fluorescence upon receipt of ultraviolet light emitted from the ultraviolet light emitting device.
The first type of white light emitting device is disclosed in Japanese Patent Laid-Open Nos. 10-093146, 10-065221, and 10-242513. The white light emitting device of this type contains an LED formed of a blue light emitting nitride semiconductor and a fluorescent material (referred to as “YAG:Ce fluorescent material”), which emits a complementary blue upon receipt of blue light emitted from the LED and which is formed of a YAG based oxide represented by (Y, Gd)3(Al, Ga)5O12 and having Ce doped in its host lattice. Particles of the fluorescent material are dispersed in an inclusion resin surrounding the LED. Furthermore, Japanese Patent Laid-Open No. 11-46015 discloses a white light emitting device in which a layer containing a non-particulate fluorescent material represented by (Re1-xSmx)3(Al1-yGay)5O12:Ce is formed on an LED.
These white light emitting devices are used in backlight for a display and an LED display device, etc. However, the emission intensity of red light emission area is not sufficient. Consequently, when the white light emitting device is used in lighting, for example, the skin color looks bluish. Since the natural color of an object cannot be faithfully reproduced as is the case may be, these white light emitting devices have a problem in color reproducing and rendering properties for use in display light for commercial goods.
The second type of white light emitting device is improved in color reproducing and rendering properties compared to the first type. Japanese Patent Laid-Open No. 2005-235934 discloses a white light emitting device using a YAG:Ce fluorescent material, which serves as a fluorescent material emitting a complementary blue upon receipt of blue light from an LED, and a red emitting fluorescent material having a solid solution of Eu in a CaAlSiN3 crystalline phase. Japanese Patent Laid-Open No. 2005-60714 discloses a white light emitting device using a YAG:Ce florescent material and a red emitting florescent material represented by Ca2-xSi5N8:Eux. Japanese Patent Laid-Open No. 2004-327518 discloses a white light emitting device using an LED emitting blue light, a europium doped yttrium/aluminum/garnet based green emitting fluorescent material and a europium doped strontium thiogallate based red emitting fluorescent material.
The third type of white light emitting device is a three-wavelength type white light emitting device, which contains an LED emitting ultraviolet light in combination with a blue emitting fluorescent material, a green emitting fluorescent material and a red emitting fluorescent material, as is disclosed in National Publication of International Patent Application No. WO98/39805. In the white light emitting device of this type, a centroclinal transparent resin, which has an ultraviolet light emitting diode arranged therein and three types of fluorescent material powders mixed therein, is formed on a transparent substrate (front panel). To the surface of the transparent resin, mirror processing is applied such that the surface works as a mirror.
These white light emitting devices employ a red emitting fluorescent material such as Y2O2S:Eu3+, which emits light when excited by light having a wavelength of 370 nm. Therefore, red light emission cannot be sufficiently obtained in a white light emitting device having an LED emitting ultraviolet light, which has an emission peak between a wavelength of 390 nm and 400 nm. In the circumstances, it has been desired to develop a red emitting fluorescent material emitting light when excited by light having a wavelength of near 390 nm. Since the visual sensitivity of red light is low compared to blue/green light, it has been desired to develop a red emitting fluorescent material emitting a further brighter light.
[Patent Document 1] Japanese Patent Laid-Open No. 10-093146
[Patent Document 2] Japanese Patent Laid-Open No. 10-065221
[Patent Document 3] Japanese Patent Laid-Open No. 10-242513
[Patent Document 4] Japanese Patent Laid-Open No. 11-46015
[Patent Document 5] National Publication of International Patent Application No. WO02/011214
[Patent Document 6] Japanese Patent Laid-Open No. 2005-235934
[Patent Document 7] Japanese Patent Laid-Open No. 2005-60714
[Patent Document 8] National Publication of International Patent Application No. WO98/39805